Process for producing laminated film and laminated sheet

Abstract: A process for producing an extrusion laminated film or sheet is disclosed, which process uses no chemical primer and comprises a plastic substrate and a resin for extrusion laminating which process uses no chemical primer and comprises: (B) melt-extruding a resin for extrusion laminating selected from polyethylene resins, polypropylene resins, ethylene-vinylester copolymer resins and ethylene-(metha)acrylate copolymer resins into a molten web at a temperature of from 180°C to 340°C and treating at least one surface of the molten web with ozone; and (C) contacting the substrate with the ozone-treated surface of the molten web obtained in step (B), and adhering the molten web onto the substrate under pressure.

Description: The present invention relates to a process for producing an extrusion laminated film and an extrusion laminated sheet. More particularly, the present invention relates to a process for producing an extrusion laminated film and an extrusion laminated sheet with at least one resin for extrusion laminating selected from the group consisting of polyethylene resins, polypropylene resins, ethylene-vinylester copolymer resins and ethylene(metha)acrylate copolymer resins, which enables production of an extrusion laminated film and an extrusion laminated sheet comprising a plastic substrate having films firmly adhered thereto without using any chemical primer agent. Production of extrusion laminated films and sheets comprising stacked different film materials such as plastic, paper, metal foils and the like having additionally other properties than those possessed by single material, for example, increased strength, gas impermeability, moisture-proofing property, heat-sealing property, improved appearance, and the like has been widely conducted and the products have primarily been used in a variety of wrappings. Such laminated films and sheets have been produced by any one of methods such as dry laminating, wet laminating, hot laminating, extrusion laminating, coextrusion laminating, and the like, selected depending upon characteristics to be utilized. Wrapping materials comprising a substrate having a heat-seal layer have been produced by the popularly used extrusion laminating method to form the heat-seal layer on the substrate which is advantageous in cost. Materials to be generally used for the heat-seal layer include polyolefin resins such as polyethylenes, polypropylenes, and ethylene copolymers, and ionomers. However, polyolefin resins have been used in large qualities from the economical standpoint. These resins are generally melt-extruded and laminated onto the adhesive surface of a substrate which has been precoated with a chemical primer to promote the adhesion between the substrate and the resins. The chemical primers to be used include organic titanate, organic isocyanate, and polyethyleneimine type adhesives. When used, these adhesives are usually diluted with an organic solvent such as toluene, ethyl acetate, methanol, hexane and the like. However, these methods using the chemical primers produce problems such as increased production costs owing to the use of expensive chemical primers, required complicated steps of applying and drying the chemical primers, scattering of organic solvents harmful to human beings into the air when the chemical primers are dried by evaporating the organic solvents therein causing environmental hazards in and around workplaces, occurrence of fire due to the use of inflammable organic solvents, and limited utilization of the end film or sheet products in food wrappings owing to the odors attributable to the residual organic solvents in the chemical primers on the products. As a method without using the chemical primers, an attempt has been made to produce a laminate by melt-kneading an ethylene copolymer produced by copolymerization of (a) ethylene, (b) unsaturated polybasic acid, and (c) an unsaturated monomer selected from lower alkyl acrylate esters, lower alkyl methacrylate esters and vinyl esters, extruding said molten copolymer at a temperature of 150 °C to 330 °C into a molten web, treating the molten web with ozone and laminating the molten web onto a substrate under pressure with the ozone treated surface of the molten web being adhered onto the surface of the substrate as described in Japanese Patent KOKAI (Laid-open) No. Hei 4-368845, wherein however only biaxially stretched nylon film was practically employed, and no practical use of ethylene-vinyl alcohol copolymers as substrate was disclosed. In addition, the methods using the ethylene copolymers containing the unsaturated polybasic acids as comonomer component and having adhesive function suffer from added production cost, lower release property in the extrusion laminating process of laminates with an increase in the amount of low melting components to limit processing temperatures as well as cumbersome exchange of resins in the extruder which may be required by the methods. Alternatively, a method for producing an extrusion laminated film by treating the surfaces of an ethylene-α-olefin copolymer molten web to be adhered onto a substrate with ozone and laminating the molten web onto the substrate without applying any chemical primer to the substrate, or a method for producing an extrusion laminated film by coextruding a modified polyolefin resin grafted with unsaturated carboxylic acids having an adhesive property onto a substrate without any chemical primer through a combination with a coextruder have been known, as disclosed in CONVERTIC (8) p. 36, (1991). However, the laminates obtained by these methods have disadvantages of insufficient adhesion strength between the laminated resin layer and the substrate limiting the area where the laminates are applied. Moreover, the method using the adhesive modified polyolefin resins grafted with unsaturated carboxylic acids requires disadvantageously the use of coextruder with an increase in production cost as well as cumbersome exchange of resins in the extruder. An object of the present invention, to overcome the aforementioned problems, is to provide a process for producing an extrusion laminated film or sheet comprising a plastic substrate and a resin for extrusion laminating selected from the group consisting of polyethylene resins, polypropylene resins, ethylene-vinylester copolymer resins and ethylene-(metha)acrylate copolymer resins, wherein the strongly adhered laminate film or sheet can be obtained without using chemical primers which may cause aforementioned many problems. Accordingly the present invention provides a process for producing an extrusion laminated film or sheet comprising an ethylene-vinyl alcohol copolymer substrate and a resin for extrusion laminating, which process uses no chemical primer and comprises (B) melt-extruding a resin for extrusion laminating selected from polyethylene resins, polypropylene resins, ethylene-vinylester copolymer resins and ethylene(metha)acrylate copolymer resins into a molten web at a temperature of 180°C to 340°C, and subjecting at least one of the surfaces of the molten web to a treatment with ozone; and (C) contacting the substrate with the ozone-treated surface of the molten webb obtained in (B), and adhering the molten web onto the substrate under pressure. Materials to be used as the substrate in the process of the present invention are ethylene-vinyl alcohol copolymers. The ethylene-vinyl alcohol may be produced by a method comprising copolymerizing ethylene and vinyl acetate, and then saponifying the product, or by a method comprising copolymerizing ethylene and vinyl alcohol. The ethylene to be used in the copolymerisation may be commercially available ethylene of a grade of usually 25 to 45 mole %. The ethylene-vinyl alcohol copolymers to be used in the present invention may have no limitation in the degree of saponification and composition of the copolymers so long as they have a performance as laminate substrate. The ethylene-vinyl alcohol copolymers may be used alone or in combination in the form of laminate films and sheets, stretched films, coatings and fabrics. Such materials may be laminated with aluminium, iron, paper or the like with the substrate being disposed on the abutting plane and the produced laminates may be used. If necessary, the surfaces of the substrates may be pretreated by corona discharge, plasma treatment, or flame treatment, or they may be pre-printed. The thickness of the substrates are not critical so long as the substrates can be processes by extrusion laminating, and preferably should be in the range of 1 to 10,000 µm, more preferably 5 to 500 µ. The resins for extrusion laminating to be used in the process of the present invention may be any one or a mixture of two or more selected from the group consisting of polyethylene resins, polypropylene resins, ethylene-vinylester copolymer resins and ethylene-(metha)acrylate copolymer resins. If necessary, other resins may be mixed in an amount of less than 50 % by weight. The process for producing the polyethylene resins is not critical, for example, they may be produced by radical polymerization or ion polymerization. The polyethylene resins include, for example, low density polyethylenes produced by radical polymerization, high density polyethylenes produced by ion polymerization, and ethylene-α-olefin copolymers produced by copolymerization of ethylene and α-olefins. The α-olefins to be used include, for example, α-olefins having 3 to 18 carbon atoms, such as propylene, butene-1, 4-methylpentene-1, hexene-1, octene-1, decene-1, and octadecene-1, which may be used alone or in combination of two or more. The ethylene-α-olefin copolymers should have an α-olefin content of, preferably 1 to 30 % by weight, more preferably 5 to 20 % by weight. The process for producing polypropylene resins is not critical, and for example, they may be produced by ion polymerization. The polypropylene resins include, for example, homopolymers of propylene, copolymers of propylene with ethylene, copolymers of propylene with butene-1, and copolymers of propylene with α-olefins. For the propylene copolymers, one or more α-olefins may be used. The propylene-α-olefin copolymers should have an α-olefin content of, preferably 0.1 to 40 % by weight, more preferably 1 to 30 % by weight. Ethylene-vinylester copolymer resins and ethylene-(metha)acrylate copolymer resins may be produced by radical polymerization, e.g., by copolymerization of ethylene with radically copolymerizable monomers. Vinyl esters for the ethylene-vinylester copolymers include, for example, vinyl acetate and vinyl propionate. (Metha)acrylates for the ethylene-(metha) acrylate copolymers include, for example, acrylates having 4 to 8 carbon atoms such as methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, t-butyl acrylate, isobutyl acrylate, and methacrylates having 4 to 8 carbon atoms such as methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, t-butyl methacrylate, and isobutyl methacrylate. One or more of these comonomers may be used. Ethylene-vinylester copolymers and ethylene(metha)acrylate copolymers should have a comonomer content of, preferably not higher than 30 % by weight, more preferably not higher than 20 % by weight. A content of comonomer component to be copolymerized with ethylene higher than 30 % by weight results in worse odor of the laminate films and sheets rendering them unsuitable for food wrapping materials as well as increased production cost, though there is no problem in the adhesiveness to the substrate, the primary object of the present invention. Polyethylene resins, ethylene-vinylester copolymer resins, and ethylene-(metha)acrylate copolymer resins should have a melt flow rate (MER) at 190 °C in the range of 1 to 100 g/10 min., and polypropylene resins should have a MER at 230 °C in the range of 1 to 100 g/10 min. The ozone-treating step (B) of the process of the present invention comprises melt-extruding a resin for extrusion laminating into a molten web at a temperature of 180 °C to 340 °C and subjecting at least one of the surfaces of the film to a treatment with ozone. The treatment with ozone may be performed by, for example, blowing a gas such as containing ozone from a nozzle or slit like blowing opening in a lower open gap disposed under a T die onto the extruded molten web. When the ozone nozzle can not be disposed under the T die, the gas may be blown onto the plastic substrate immediate before laminating under pressure. The amount of ozone to be blown should preferably be not less than 1.0 mg per square meter of the surface of the extruded molten web which is being transferred, more preferably not less than 10.0 mg/m 2 . The resins for extrusion laminating are melt-extruded into a molten web at a temperature of 180 °C to 340 °C . A temperature lower than 180 °C may result in poor stretchability of the molten webs, make it difficult to produce uniform thickness of molten web, and cause insufficient adhesive strength with the plastic substrate. On the other hand, a temperature higher than 340 °C may produce severer oxidation of the surfaces of the molten webs and worse odor of the webs. Preferred resin temperature is from 210 to 330 °C . The pressurized laminating step (C) in this embodiment comprises contacting the substrate with the ozone-treated surface of the molten web obtained in (B) the ozone-treating step, and adhering the molten web onto the substrate under pressure using known extrusion laminater. In this embodiment (A) a surface oxidizing step such as (a 1 ) corona discharge treatment and the like may be provided additionally to the aforementioned two steps. The term "(A) a surface oxidizing step" refers, for example, in the case of (a 1 ) corona discharge treatment, to a step comprising treating at least one of the surfaces of the substrate with corona discharge. The corona discharge treatment (a 1 ) may be conducted by passing the plastic substrate through a corona atmosphere generated with known corona discharging apparatus. From the standpoint of achieving a high level of adhesive strength, preferably the corona discharge density should not be less than 10 W·min./m 2 , more preferably not less than 20 W·min./m 2 . Moreover, the corona discharge treatment step and the pressurized laminating step (C) as described above should preferably be arranged in-line so that the plastic substrate after the corona discharge treatment step can be transferred to the pressurized laminating step (C). In this way a high level of adhesive strength can be developed and undesirable blocking of the plastic substrate can be prevented. In the present invention, the resin extrusion laminated onto the plastic substrate may be applied to the heat-seal layer of the laminate films or sheets or to the intermediate layer of the laminate films or sheets depending upon the resin's functions such as easily heat-sealable property and moisture proofing property. Moreover, the present invention can apply to a sadwich type extrusion 5 laminating method to form a laminate. To the resins for extrusion laminating used in the present invention, there may be added known additives such as anti-oxidant agent, anti-blocking agent, weather-resisting agent, neutralizing agent, flame retarding agent, antistatic agent, anti-fogging agent, sliding agent, dispersant, pigment, organic or inorganic fillers. The extrusion laminated films and sheets produced by the process of the present invention have utility in wrapping materials such as food wrapping materials and medicine packaging materials as well as industrial materials. The present invention is further illustrated with reference to the following examples. Examples 1 to 4 A low density polyethylene (LDPE, available from Sumitomo Chemical Industry Co. Ltd., under tradename "SUMIKATHENE", L716-H MFR 7 g/10 min., Density: 0.919 g/cm 3 or a linear low density polyethylene (LLDPE, available from Sumitomo Chemical Industry Co., Ltd., under tradename "SUMIKATHENE a" CS8026 MFR 10 g/10 min., Density: 0/914 g/cm 3 ) was melt-kneaded with an extruder having an aperture diameter of 65 mm and extruded through a T die at a resin temperature of 315°C or 295°C into a molten web having a width of 450 mm a thickness of 40 µ at an extrusion laminating speed of 80 m/min., and an ozone-containing air was blown to the surface of the molten web to be adhered onto the substrate through a nozzle disposed at a distance of 30 mm from the bottom of the die to achieve treatment of the contact surface of the molten web with ozone. The amount of ozone used for the treatment was 37 mg/m 2 . Then the molten web was laminated onto a 15 µ biaxially stretched ethylene-vinylalcohol copolymer film to produce a laminate film. The produced laminate film was evaluated for various properties as described below. The results of the evaluation and the processing conditions are given in Table 1. It can be seen from the results that all the examples satisfying the conditions of the present invention could afford satisfactory film adhesion strength. Example 1 2 3 4 Plastic substrate EVOH EVOH CEVOH EVOH Resin for extrusion laminating LDPE LDPE LLDPE LLDPE Process conditions Melt-extrusion temperature (°C ) 315 315 295 295 Speed (m/min.) 80 80 80 80 Thickness (µ) 40 40 40 40 Corona discharge treatment Treatment density (W·min/m 2 ) - 15 - - Ozone treatment Treatment amount (mg/m 2 ) 37 37 37 37 Evaluation Peel adhesion strength (g/15mm) Could not be separated 460

Claims: A process for producing an extrusion laminated film or sheet comprising an ethylene-vinyl alcohol copolymer substrate and a resin for extrusion laminating, which process uses no chemical primer and comprises (B) melt-extruding a resin for extrusion laminating selected from polyethylene resins, polypropylene resins, ethylene-vinylester copolymer resins and ethylene-(metha)acrylate copolymer resins into a molten web at a temperature of 180° to 340°C, and subjecting at least one of the surfaces of the molten web to treatment with ozone; and (C) contacting the substrate with the ozone-treated surface of the molten web obtained in step (B), and adhering the molten web onto the substrate under pressure.

Inventors: GOTOH YUJI, HAYASHIDA HARUO, ISHIBASHI FUMIO, KUBO KOHJI, TAKAHATA HIROAKI, GOTOH, YUJI, HAYASHIDA, HARUO, ISHIBASHI, FUMIO, KUBO, KOHJI, TAKAHATA, HIROAKI
Applicants: SUMITOMO CHEMICAL CO, SUMITOMO CHEMICAL COMPANY, LIMITED
Application year: 1994
Main classification: B32B31
IPC codes: C08J5, C08J7, B29C47, B29C59, B29C43, B29C35, B32B37